With introduction of idling stop cars or the like, liquid lead storage batteries have been increasingly used in a partially charged state (Partial State of Charge; PSOC). For use in PSOC, the liquid lead storage battery often ends its life due to sulfation of a negative electrode. Negative electrode sulfation is a phenomenon in which a negative active material is changed into lead sulfate which is bulky and difficult to be reduced to metal lead, so that a storage battery loses its capacity. It is known to be effective to add fine and electrically conductive carbon such as carbon black to a negative active material in a large amount for suppressing negative electrode sulfation (Patent Document 1; JP 3185508 B). For example, Patent Document 1 discloses that negative electrode sulfation can be suppressed by including 0.4 to 7.5% by mass of carbon in a negative active material of a sealed (valve regulated) lead storage battery. In the sealed lead storage battery, an electrolyte solution is retained in a retainer mat, and therefore has low fluidity. Therefore, such a problem hardly occurs that carbon flows out from the negative active material into the electrolyte solution and spreads.
When a large amount of carbon is included in a negative active material in a liquid lead storage battery with an electrolyte solution existing in a flow state, carbon, which is in the form of fine particles, flows out into the electrolyte solution to make the electrolyte solution turbid. Also, since a difference in specific gravity between carbon and the electrolyte solution is relatively small, carbon is blown up by convection of the electrolyte solution and a generated gas, and deposited on the upper part of an element, so that a short circuit may be caused in the worst case. Therefore, in the liquid lead storage battery, it is difficult to include 0.3% by mass or more of carbon in the negative active material.